Flexible printed circuits and rigid-flexible printed circuits are used in many applications where at least certain parts of the circuits need to be installed in a bent state. Flex circuitry incorporates metal lines sandwiched between non-conductive flexible layers of the flexible printed circuit. However, as more layers of metal and non-conductive substrates are added to the sandwich, the flexible printed circuit becomes less flexible. In addition, attempts to add electrical or electronic devices require the mounting of components onto the surfaces of the flexible circuit. The surface mounted components, i.e., surface mounted devices (SMDs), make the flexible circuit assembly even more rigid and less flexible, and substantially increase the height of the flexible circuit assembly.
Electronic systems are often separated onto two or three circuit boards. Rigid printed circuit boards (PCBs) are used to mount and support the electronic devices and include many copper layers to interconnect the respective SMDs. Separate flexible interconnects are used to provide interconnection between the individual rigid PCBs. Also, the flexible circuits are typically structured with two or more metal layers. Thus, the system is somewhat flexible in the interconnect flex circuit regions, but rigid where components are mounted. As a result, the multi-component system is not optimized for size and weight parameters. Furthermore, the combined PCB-flex manufacturing processes are complex and expensive. Rigid flex technology employs methods to thicken and stiffen a region of the flexible circuit to provide a region that is mechanically rigid to accommodate fragile components, e.g., surface mount devices and through-hole connectors. The process for mounting SMDs is likewise complex and less cost effective. For example, over-molding of devices such as semiconductor circuits, requires additional assembly and packaging process steps. Devices are diced from a wafer to form a die are first assembled into a packaged device, and the packaged device is then soldered onto a PCB to complete assembly.
In addition, the aforementioned PCB substrates, rigid-flex substrates, and flexible substrates are poor conductors of heat. Therefore, when heat generated by the mounted device is excessive, e.g., in the case of power circuits, microprocessors, and light-emitting devices, more expensive thermally conductive substrates accompanied with the attachment of a bulky conducting heat sink are required. Without the attachment of such heat sinks, many devices, especially power devices and microprocessors, cannot be fully tested. The heat sink is attached to the underside of a metal core (MC) substrate, MC-PCB, or on top of the packaged SMD to transfer heat away from the mounted device. The heat sink is typically metallic copper or aluminum fins and its attachment to the substrate or package makes the assembly bulky, heavy, and very inflexible. In addition, FR4 and adhesive materials that are conventionally used for PCBs cannot be processed above 270° C. and higher melting, lead-free solders require bonding at higher temperatures that can decompose FR4 and the adhesive material. Also, FR4 is made of toxic materials and cannot be used for implant electronics, such as pacemakers.
Moreover, for electro-magnetic shielding of the aforementioned PCB substrates, rigid-flex substrates, and flexible substrates for electromagnetic interference (EMI) protection, additional metal casings around these PCBs are used that add increased cost, weight, and inflexibility to the electronic systems, and can also further decrease the extraction of thermal energy from the circuits, and may require additional sophisticated heat sink structures.
Therefore, despite all the existing flexible circuit technology, in light of the above deficiencies of the background technology, what is needed is an adaptable and cost-effective method of manufacturing flexible circuit assemblies that permits mounting of an increased number of power devices in a cost effective weight and space saving manner, transfers heat efficiently away from heat generating devices, and allows the use of highly effectual automated roll-to-roll manufacturing concepts.